Why did the US Army's Chief warn against "little green men"?

"Youll be dealing with terrorists, youll be dealing with hybrid armies, youll be dealing with little green men, youll be dealing with tribes, youre going to be dealing with it all, and youre going to be dealing with it simultaneously, United States Army Chief of Staff Gen. Mark Milley said in a speech last week at Norwich University.

Ummm... what?

Milley's comment fueled conspiracy theories across the Web.

The Army later weighed in though.

"It does not mean fighting troops from outer space, the officials said. According to the Army Times, "The phrase 'little green men' typically refers to foreign troops or paramilitary forces who dress in green attire instead of traditional military gear."